Question on solar, battery voltage and fridge

[vagari]

This is a little long.

First ... I bought the Renogy 400 watt premium kit on Amazon with MPPT controller. I bought an MT-5 display. I bought two 12V Duracell 100AH AGM's from Sam's (200AH combined). I have the panels installed on my roof racks and everything wired up. I wired in some 12V power outlets. Everything works great. It's great feeling when you do it all yourself for the first time.

Now ... It was 65 degrees outside. I plugged in my Whynter 65 quart fridge (~4.5A when running). After it went from 69 down to 35 degrees I waited for the light on my MPPT to start flashing green indicating full charge on batteries. The MT-5 displayed 100% capacity (5 of 5 bars) and I thought it also said 14.4V on batteries (I'll double check today if sun comes out). I left fridge on for 6 hours from 4pm to 10pm. At 10pm the MPPT light was then solid green meaning batteries not low (orange) but not 100% either. The MT-5 then showed 12.7V on batteries and that the remaining capacity was 60% (3 out of 5 bars). I have seen some posts/pages that 12.7V means near 100% charged batteries.

Questions:
Did I use 40% of batteries for fridge for 6 hours?
Did my batteries go from 14.4V to 12.7V in 6 hours?
Was the 14.4V on batteries because they were charging and then 12.7V when not charging?
After 6 hours of fridge use would 12.7V on batteries mean they are still near full charge?
The MPPT has 3 battery types: Sealed lead acid, gel and flooded. Are AGM's sealed lead acid?

 

[Optimistic Paranoid]

The inherent chemistry of lead acid is such that a single, fully charged cell is 2.1 volts (rounded off). A "12 volt" battery is six such cells in series, so a fully charged battery is 12.6 - 12.7 volts.

Voltage is actually pressure, so to force electrons into a battery, the charging source must provide more voltage than the battery contains. Any time you're seeing voltage higher than 12.7 volts, you're seeing the voltage of the charging source.

AGM batteries are sealed lead acid.

It's highly unlikely that the batteries were fully charged when you got them, and definitely possible that the system didn't get them to full charge on the first day.

I'd give it a week for the system to stabilize before trying to draw any conclusions.

Regards
John

 

[Zil]

I don't know exactly how that monitor works, but most have to be "zeroed" to your battery bank when the bank is fully charged. As posted above, your batteries were not fully charged off the shelf.

 

[akrvbob]

Batteries are 12 volt in name only. By that I mean that a battery that reads 12 volt is very low, almost empty. A full battery should read 12.6-12.8

During charging, batteries need to be charged at much higher rate than 12 volt and 13 to high 14s is common.

It's inherent in the prorperites of batteries that when they are under a charge or discharge, they give a false reading because of something called a "surface charge" on the acid bath inside. The higher the charge, the more inaccurate they are.

So a full battery that is being charged by the sun should read about 14.4.
After the sun goes down, if it is not being drawn, on it will read about 13.2
By mid-evening it should read 12.7
By morning it should read 12.6

That's all assuming it's not being drawn on. If something is drawing from it , the reading will reduce the numbers because of surface charge and it will also go down because the battery is going down.

Let's assume by bed time the battery reads 12.3 and you turn everything off and go to bed. The next morning before the sun hits the panel it will read 12.4. That is pretty much the accurate state of it's charge.

Here's the bottom line:

Under charge the battery gives a falsely high reading
Under discharge it gives a falsely low reading.
After resting for a few hours it gives a true reading.

Bob

 

[SternWake]

Most every chargng source out there is simply wrong when they flash the full charge light.

AGM batteries are sealed lead acid batteries, and while many people will say AGMS need 14.x volts for absorption and to be floated at 13.x volts, the ranges listed among the various AGM battery manufacturers can range widely.

Some AGM's have fully charged resting voltages over 13 volts. My Northstar AGM is 13.06v

Figureing out state of charge of any battery is not as simple as reading voltage or looking at a light some charging source or monitor displays.

Even the respected amp hour counters, which are the best ways to have an idea, are not 100% accurate. They need to be re zeroed when it is known that the batteries are indeed truly fully charged.

With Flooded batteries one can use a Glass turkey baster hydrometer to determine when the batteries reach full charge
AGM;s are much more difficult and require either an amp hour counter, or an Ammeter.

With an Ammeter, when a hundred amp hour capacity AGM battery can no longer accept more than 0.5 amps at the manufacturer listed absorption voltage( at 77f degrees) then one can consider it fully charged.

The solar controller has very little idea of battery state of charge. They are programmed with a a one size fits all algorithm, or in the case of your controller, 1 of 3 sizes fits all, and they follow it, and with enough sunlight and time, they flash the magical full charge, warm and fuzzy, feel good green light, and it is not indicative of a true full charge. It might get close, it could be 20% out. If other charging sources are present, like the alternator, it might see voltage higher than it itself would allow and instantly proclaim a full charge and the battery could be in the 55% range still.

Unfortunately knowing true state of charge of batteries is not easy to figure out. Trusting any given source to tell you can be unwise, and possibly unwise in the extreme. Lots of information online and in forums on this topic is simply wrong, and doing a disservice to the reader.

but thankfully they are just batteries, and just rented, and when they fail prematurely through chronic undercharging one can goto the store and stamp their foot and hope the manager applies some warranty to the user induced failure.

I congratulate you on your suspicions about the full charge light lying to you. If you can remove all loads from the battery for a day or morem and let the solar do its thing, then you can assume that you are starting a discharge cycle from 100%. If you monitor ho =w much you use over how long, and take voltage readings, then you can notice the trends and tendencies of voltage under load, when under sunlight and make an increasingly educated guess at to battery state of charge.

Battery monitors like a Trimetric do the amp hour counting for you, but cannot be considered 100% accurate. they too need to be re zeroed when it is known that the batteries are fully charged and that can be determined by the amount of amps they accept at 14.x charging volts.

amazing that ancient technology, lead acid batteries, their care and charging can be so complicated, but that is the way it is.

Much easier to remain ignorant of it and just have the funds to replace batteries, but for those of us who are not well funded, or dislike the disposable nature of our culture, we strive to get the most from a battery, and that requires effort and skills and the ability to not trust blinking green soothing, warm and fuzzy, feel good lights.

But that effort might not really be worth it.

Keep in mind that what some might say works just fine for them, really means is just that the battery has not yet failed to meet their needs, the capacity of their battery is still enough to meet their needs. This does not mean that the batteries are still "like new' as many might proclaim.

As measuring actual remaining capacity in a battery is even more complicated than measuring state of charge of a battery in use, accurately.

Voltage is ONLY ACCURATE on a rested battery, one which has not seen charging or discharging currents for many hours. The time at rest is different for each and every battery, and will change as the battery ages.


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